Tuning by hear can take some practice and these days with easy access to electronic tuners its really a skill that’s not as important as it used to be. However I think it’s important to at least understand how this works, and with the help of an electronic tuner your ears will be begin to hear whats in or out of tune.
Tuning “by ear”simply means that you listen and adjust according to what you hear, instead of using a tuner or app. This method is the most common method of tuning guitar by ear.
With this method, you effectively “tune the guitar to itself”. You either get one string in tune from an external reference point, or just decide that one string is close enough. You tune the other strings from this starting point. The end result is that the guitar may be in tune with itself (meaning it sounds “right”), but it may not be in tune with what the larger world considers “in tune”.
This is why using an electronic tuner is probably the simplest solution
Firstly the E 6th string has to be in tune or at least you have to start the process from that string. So once you have the pitch of that string you can tune the others in sequence
Here’s the process, assuming the 6th string is in tune to begin with:
You’ll notice when the two notes are out of tune that there is a “warbling” sound. This warble is created by the two different vibration rates of the strings. The two strings cause this warble by vibrating at different rates (i.e. different pitches). The warble will slow as you approach correct tuning. “Well in tune” sounds a steady pitch, with no warbling between the two notes.